The present invention relates to a reinforced cement sheet product which does not contain asbestos and which can be fabricated on a Hatschek machine.
By way of background, in the past asbestos-cement compositions have been formed into sheets on a Hatschek machine for subsequent formation into such products as siding shingles, flat sheets, corrugated sheet products, and the like. These sheets generally contained Type 1 Portland cement binder in the amount of about 58%, silica and/or filler in the amount of about 27%, and asbestos fibers in the amount of about 15%. The products had many desirable qualities including high strength, and exterior durability, easy application, and ability to be colored in various colors. In addition, the existence of the asbestos permitted the product to be formed expediently and economically on a Hatschek machine. However, in recent years governmental regulations have restricted the use of asbestos due to its carcinogenic effects. Therefore, attempts have been made to find substitutes for asbestos. One such attempt is disclosed in U.S. Pat. No. 4,040,851 which utilizes cotton fibers instead of asbestos fibers, along with other components in addition to Portland cement and silica. However, in the past it has been found that formulas containing asbestos substitutes do not have the properties of asbestos which are conducive to the expedient formation of the desired product on a Hatschek machine. More specifically, it has been found that cement and silica and/or filler slurry containing fibers other than asbestos will not form properly on the cylinder of a Hatschek machine for two reasons, namely, (1) there is too rapid a drainage of water from the non-asbestos mat which is formed on the Hatschek cylinder and (2) there is an excessive loss of cement and silica and/or filler fines thru the cylinder screen which causes problems in handling the discharge water in the normal recirculating system and which causes the resulting product to have reduced strength. It has also been found that various synthetic fibers, such as polyethylene, polypropylene, etc. and some natural fibers cannot be used instead of asbestos because they will not disperse properly in the cement-silica water slurry. In this respect, they tend to float to the top of the slurry because they have a lower specific gravity than water, and therefore they will not form a homogeneous mat on the cylinder of a Hatschek machine. It is with overcoming the foregoing problems that the present invention is concerned.